Many vessels in animals transport fluids from one bodily location to another. Frequently, fluid flows in a unidirectional manner along the length of the vessel. Varying fluid pressures over time, however, can introduce a reverse flow direction in the vessel. In some vessels, such as mammalian veins, natural valves are positioned along the length of the vessel and act as one-way check valves that open to permit the flow of fluid in the desired direction, and quickly close upon a change in pressure, such as a transition from systole to diastole, to prevent fluid flow in a reverse direction, i.e., retrograde flow.
While natural valves may function for an extended time, some may lose effectiveness, which can lead to physical manifestations and pathology. For example, venous valves are susceptible to becoming insufficient due to one or more of a variety of factors. Over time, the vessel wall may stretch, affecting the ability of the valve leaflets to close. Furthermore, the leaflets may become damaged, such as by formation of thrombus and scar tissue, which may also affect the ability of the valve leaflets to close. Once valves are damaged, venous valve insufficiency may be present, and can lead to discomfort and possibly ulcers in the legs and ankles.
Current treatments for venous valve insufficiency include the use of compression stockings that are placed around the leg of a patient in an effort to force the vessel walls radially inward to restore valve function. Surgical techniques are also employed in which valves can be bypassed or replaced with autologous sections of veins with competent valves.
Minimally invasive techniques and instruments for placement of intraluminal medical devices have developed over recent years. A wide variety of treatment devices that utilize minimally invasive technology has been developed and includes stents, stent grafts, occlusion devices, infusion catheters and the like. Minimally invasive intravascular devices have especially become popular with the introduction of coronary stents to the U.S. market in the early 1990's. Coronary and peripheral stents have been proven to provide a superior means of maintaining vessel patency, and have become widely accepted in the medical community. Furthermore, the use of stents has been extended to treat aneurisms and to provide occlusion devices, among other uses.
Recently, prosthetic valves have been developed that use a support structure such as a stent. Frequently, a graft member is attached to the support structure and provides a valve function to the device. For example, the graft member can be in the form of a leaflet that is attached to a stent and movable between first and second positions. In a first position, the valve is open and allows fluid flow to proceed through a vessel in a first direction, and in a second position the valve is closed to prevent fluid flow in a second, opposite direction. An example of this type of prosthetic valve is described in commonly owned U.S. Pat. No. 6,508,833, to Pavcnik for a MULTIPLE-SIDED INTRALUMINAL MEDICAL DEVICE, which is hereby incorporated by reference in its entirety. In other examples of prosthetic valves, a tube that terminates in leaflets is attached to one or more support structures to form a valve. The leaflets open to permit fluid flow in a first direction in response to fluid pressure on one side of the leaflets, and close to prevent fluid flow in a second, opposite direction in response to fluid pressure on opposite sides of the leaflets. An example of this configuration is provided in U.S. Pat. No. 6,494,909 to Greenhalgh for AN ENDOVASCULAR VALVE, which is hereby incorporated by reference in its entirety.
Support structures in some medical devices, such as stent grafts, can exert an outwardly-directed radial force upon an interior wall of a body vessel in which such a medical device is implanted. The exertion of such a force has potential to induce an inflammatory response or other effect within the body vessel and/or the surrounding tissue. Accordingly, medical devices with constrained expansion are desirable.